1. Field of the Invention
The present invention relates to vertical takeoff and landing (VTOL) aircraft. More particularly, the present invention relates to a system and method for inbound transition control for a tail-sitting VTOL aircraft.
2. Background Information
In a “tail sitter”-type vertical takeoff and landing (VTOL) aircraft, the fuselage is horizontal for normal flight and vertical for hover, takeoff and landing. The same propulsion system is used for forward flight and for hover. The propulsion system can either be a ducted fan or an external propeller/rotor. The body uses control surfaces that interact with the internal and/or external air flow to produce control moments that control the body attitude. At low flight speeds, the predominant air flow over these control surfaces is the air blown back by the propeller or fan that is producing the aircraft's thrust. The body may or may not have fixed wings or tail surfaces attached. The body can also have some type of “freewings” attached. The freewings are wing panels that are allowed to freely pivot about an axis that is substantially spanwise with respect to the fuselage of the aircraft. Freewings are described in, for example, U.S. Reissue Pat. No. RE36,487 to Wainfain, U.S. Pat. No. 5,340,057 to Schmittle, and U.S. Pat. No. 5,395,073 to Rutan et al, the entire contents of each of which are incorporated by reference herein. The wing panels have control surfaces mounted on them that control the angle of attack and thus the lift coefficient of the freewing panels. The panels automatically adjust themselves so that they maintain a constant angle of attack regardless of the flight path or wind gusts.
Alternatively, the freewing panels can be actively driven by actuators in a mode that emulates a freely pivoting wing panel. The freewing panels are located entirely outside of the air flow of the propulsion system. In the case of ducted fan propulsion, all of the ducted fan aircraft's high speed air flow is inside the duct, so the free wings can be mounted anywhere on the outside the duct. In the case of a propeller, there are fixed wings attached to the body, with the wing span at least to the outer edge of the propeller slipstream, and only the wing panels outboard of the duct are pivoted as freewings. As known to those of ordinary skill in the art, a ducted fan VTOL aircraft with wings is much more efficient in horizontal flight than one that is just a duct. The area and span of the wings allows efficient low speed loiter, increases the maximum altitude and can increase the maximum level flight speed. However, there are several problems with conventional ducted fan VTOL aircraft.
For example, conventional ducted fan VTOL aircraft are sensitive to gusts while hovering. A tail sitter VTOL aircraft with wings that are predominantly vertical in the hover mode, either with fixed wings or with freewings, is extremely difficult to hover, takeoff or land in gusty winds. The total area of the wings is exposed to gusts, thus producing large forces and moments that the control system must overcome. In addition, these undesired loads can change dramatically depending on the orientation of the aircraft relative to the wind direction. Thus, the sensor payloads on the aircraft cannot be aimed in desired directions while the aircraft is hovering.
In addition, these type of aircraft require high power to hover in wind. A VTOL tail sitter aircraft with predominantly vertical wings has a very large area exposed to even a steady wind. Due to the high angle of attack of the wings, the wings will be fully stalled. If the aircraft must hover over a given location in wind, the fully-stalled wings have very high drag forces in the horizontal direction, while producing little or no lift. Thus, the main body must tilt enough so that the main thruster is producing enough horizontal thrust to overcome the high drag. The tilt reduces the available vertical thrust, thus requiring the engine to be operated at a higher power setting. Alternatively, the weight of the aircraft must be reduced by reducing fuel or payload, so that the engine has enough thrust to keep the airplane in the air.